omer berk berkalp air permeability & porosity in...

81FIBRES & TEXTILES in Eastern Europe July / September 2006, Vol. 14, No. 3 (57)

n IntroductionPorosity and air permeability are vital properties in some end-use applications such as filtration, thermal insulation and fluid barriers. The application of various permeability tests to textiles and textile materials has been practiced in the industry for years, and determin-ing permeability and porosity have long been subjects of interest in this field. Several papers have been published on the measurements of both properties for the nonwovens. Since nonwoven structures are being increasingly used in many technical applications, it is very important for the producer to understand specific properties such as permeability in detail. This paper gives a brief litera-ture review on the relationship between structural properties, porosity and per-meability of nonwovens, and intends to find answers especially concerning the relationship between hydroentanglement energy and air permeability in spun-laced nonwovens.

Previous studies investigated the rela-tionship between air permeability and structural characteristics of nonwovens such as fabric weight, thickness, density and fibre diameter [1 - 6]. It has generally been demonstrated that air permeability decreases as thickness, weight or fabric density increase. However in some stud-ies, it is found that there was a non-linear relation between the properties, whereas some other studies show them to be lin-ear. It was demonstrated in one paper that fabric weight is the most significant factor for air permeability, more than thickness, density and fibre diameter [5]; by using the same data, fabric density is

Air Permeability & Porosity in Spun-laced Fabrics

Omer Berk Berkalp

Istanbul Technical University Textile Technologies and Design Faculty,

Textile Engineering DepartmentInonu cad. No.87 Gumussuyu/Beyoglu

34437 Istanbul, TurkiyeE-mail: [email protected]

found to be the second most important factor after fabric weight [2].

The hydroentanglement process is very suitable way of converting not only the textile fibres but also new-generation and high-performance fibres and their blends into nonwovens without damaging them, and without the need for a binder. The physical characteristics of hydroentan-gled nonwoven fabrics, such as soft-ness, flexible hand, high drape and bulk, conformability and mouldalibity, high strength without binders and delamina-tion resistance, make this process unique among the processes for nonwovens [7]. The characteristics of fabrics can be engineered according to the end-use requirements.

The amount of energy delivered to the web is a crucial parameter influencing the fabric structure and properties since it affects the completeness of fibre entan-glement. ‘Completeness’ is a term that is defined as the portion of fibres that are tied together. Water pressure is another parameter related to fabric energy intake. Several water pressure levels are used [8].

Up to a certain point, higher energy lev-els result in stronger fabrics. Addition-ally, shifting the balance of energy input to the second side (low specific energy ratio) can significantly decrease fibre loss. The benefits of different energy ratios are not so clear [9]. Higher water pressure machines are mostly used, since energy can be delivered into a web with fewer water needles and less water con-sumption, which is more economically beneficial [8].

As the target weight and density of product increases, so does the water en-ergy requirement, to a point where web penetration can no longer be achieved without major fibre damage. This results in a fabric of skin-core effect, with fibres bonded at the surfaces but with little or no entanglement at the centre. Such fab-rics easily delaminate and have no com-mercial value. By selecting the optimum orifice size, shape, concentration and en-ergy distribution, it has been possible to gradually build up the entanglement with heavy weight webs, to produce unique fabrics with a dense, uniform structure throughout the cross-section [10].

For lightweight fabrics, the industry trend is to apply the entanglement energy in the form of a multitude of injectors, working at comparatively low pressure. Energy applied in this way to heavy weight webs results in good surface entanglement but weak delaminating core structures. Another basic process parameter which influences the fabric is the speed of the line. If a constant amount of energy is be-ing delivered to a fabric, the speed of the fabric determines how much energy will be absorbed per fabric unit area. Logi-cally, the higher the line speeds, the less the energy that is absorbed by the fabric, and thus the lower the fabric strength that is achieved.

The total specific energy of water ap-plied to the fibre web is a key process variable responsible for the efficiency of the transfer of the patterns onto the fabric surface. Naturally, the energy will also have a profound effect on the physical and structural properties of the fabrics.

AbstractThe relationship between air permeability and fabric characteristics such as fabric weight, thickness and density in a variety of nonwoven types has already been investigated. The effect of porosity on air permeability has also been thoroughly evaluated in numerous research works. However in this paper, we report the effects of the specific energy in hydroentan-glement on the air permeability of the resultant fabric. It is shown that within the ranges of the measurements, energy is not the only factor that affects the permeability. Although the same web weight is used in all trials, fabric weights and thicknesses did not show any clear or linear decrease when the energy given to the fabric was increased. The alteration of web weights and thicknesses under enhanced energy shows that entanglement mecha-nism in these nonwovens is not not as simple as we had expected. It is thought that the flow corridors in nonwovens are certainly very complex. This increased intricacy may be due to the randomness of nonwoven structures or the method of bonding such as the random hydroentanglement of spun-laced fabrics.

Key words: air permeability, specific energy, spun-lace nonwovens, porosity.

FIBRES & TEXTILES in Eastern Europe July / September 2006, Vol. 14, No. 3 (57)82 83FIBRES & TEXTILES in Eastern Europe July / September 2006, Vol. 14, No. 3 (57)

The energy transferred to the web can be controlled by1. manifold pressure,2. residence time,3. number of manifolds and passes.

As the target weight and density of prod-uct increases, so does the water energy requirement, to the point where web pen-etration can no longer be achieved with-out major fibre damage. The theoretical simplified formula for specific energy, E, in Jkg-1 applied to the fibre web by water jets in a manifold is given by [11]:

(1)

where Cd is the orifice discharge coef-ficient (assumed 0.7), d is the diameter of the jet orifice, m; N is the number of jets/m per manifold; P is the water pres-sure Nm-2 in the manifold; ρ is the water density, kgm-3; W is the basis weight of the web, gm-2; and S is the line speed in mmin-1. The total specific energy (Et) ap-plied to the fabric, Jkg-1 is calculated by Equation (2):

Et = (2)

where Eij is the specific energy applied to the fibre web by manifold, j (j = 1, 2, …. m), calculated by Equation (1), in the pass i (i = 1,2,…. p).

It is logical to expect that structural characteristics have an impact on air per-meability; this is dependent on porosity. However there is limited experimental evidence in the literature that these properties correlate. It is also certainly true that from the practical point of view of the non-woven fabric manufacturer, it is generally difficult to manipulate web weight for various reasons such as price, weight specification, etc. so that the manufacturer can select perhaps the most economic combination for a given end use. In the case of spun-lace produc-tion, for example, by changing the type of structure or by altering the web density (perhaps by hydroentangling to a greater or lesser extent), it may be possible to use a lighter and hence cheaper web, to give identical air permeability to that required for any particular application. There has been very limited study on spun-lace fab-rics in open literature; therefore in this study, we have aimed to investigate the effect of hydroentangling energy on the air permeability of spun-laced fabrics.

n Materials and methodsThe samples were produced at the NC State College of Textiles’ Nonwoven Laboratory. 1,5 dpf PET and Bleached cotton were used as the materials. The polyester and bleached cotton fibres were first opened, carded (by a flat top card) and cross-lapped in order to obtain webs with a bimodal fibre orientation. After form-ing the web, a pilot-scale Honeycomb hydroentangling machine was used to produce fabrics under varying pressures and numbers of passes. The speed of the line was 30 feet/min-1 (9.14 m/min-1). The machine has three manifolds; the pressure of each manifold can be inde-pendently controlled. The jet strips have two lines of orifices. The orifice diameter is 0.127 mm and the density of the jets is 16 orifices/cm. The pressure can be as low as 1.3 MPa for pre-wetting, and as high as 11 MPa for hydroentangling.

We produced fabrics at 16 (for PET) and 15 (for Cotton) different specific energy levels (by using various jet pressures and passes). The experimental design is given

in Table 1. The pressure values in Table 1 are given in psi, since the machine is adjusted in BS units. All the calculations are made in BS units and then converted to SI units.

All the fabrics were manufactured from the same web with 50 gm-2 nominal weight. All specific energy levels (calcu-lated from the equation above) and fabric properties measured are given in Table 2.

Air permeability was measured using a Frazier high-pressure air permeometer, as specified in ASTM D7371, using a pressure differential of 12.7 mm of wa-ter. A porometer was used to measure pore size characteristics as specified in ASTM E1294-89. ASTM E1294-89 is a liquid extrusion method of analysis for measuring the porosity of membrane filters which is also used effectively to evaluate the porosity of textile materials, and hence nonwovens (Figure 1).

In determining porosity, tests are carried out with dry and wet samples. The speci-men is wetted first with a liquid of low sur-

Table 1. Experimental design; the pressure in the table are given in psi as the machine has been adjusted in BS units. All calculatuions were solved in BS units and next changed to SI units.

Sample designa-

tionWeight, gm-2

Sample designa-

tionWeight,

gm-2 Passes

Pressure, psi Specific energy, 103 x kJkg-1

for Manifold #1 #2 #3 Cotton PET

Cot.1 51.7 PET 1 47.31 200 400 400

1.94 2.121 400 400 400

Cot.2 44.2 PET 2 47.11 200 400 400

4.82 4.513 400 400 400

Cot.3 53.9 PET 3 36.81 200 400 400

6.04 8.855 400 400 400

Cot.4 48.6 PET 4 46.31 200 400 400

9.00 9.467 400 400 400

Cot.5 37.2 PET 5 46.1

1 200 400 40014.8 11.95

9 400 400 400Cotton is destroyed - PET 6 44.8

1 200 400 400*** 14.8

11 400 400 400

Cot.7 47.7 PET 7 51.91 200 800 0

2.66 2.251 0 800 0

Cot.8 44.6 PET 8 49.21 200 800 800

5.87 5.441 800 800 800

Cot.9 38.0 PET 9 45.91 200 800 800

12.34 11.813 800 800 800

Cot.10 51.2 PET 10 47.31 200 800 800

18.53 18.185 800 800 800

Cot.11 49.8 PET 11 45.71 200 800 800

24.91 24.557 800 800 800

Cot.12 48.1 PET 12 51.51 200 1200 0

4.42 4.031 0 1200 0

Cot.13 45.3 PET 13 49.01 200 1200 1200

10.30 9.881 1200 1200 1200

Cot.14 39.1 PET 14 47.71 200 1200 1200

22.09 21.583 1200 1200 1200

Cot.15 37.4 PET 15 43.81 200 1200 1200

33.82 33.295 1200 1200 1200

Cot.16 46.9 PET 16 44.0 1 200 1200 1200 45.38 44.997 1200 1200 1200


face tension (Silwick (19.8 dynes per cm surface tension)), and then extruded un-der increased air pressure. The maximum pore size is determined according to the first air flow, which is characterised as the bubble point. As smaller pores are emptied, air flow through the sample increases, and the air flow is recorded as a function of air permeability. All these results are compared with that of the dry sample. The following data is used to determine the pore size distribution: the variation of flow rate with pressure for a dry sample, and a half-dry curve which is computed from the dry curve to yield half of the flow rate through the dry curve at a given differential pressure. The inter-section of the wet curve and the half-dry curve gives the mean flow pressure that

corresponds to mean pore diameter [12]. Pores smaller than the mean pore diam-eter are responsible for half of the flow.

A theoretical and calculated value for po-rosity can be determined; this is defined as the ratio of air space to the total fabric volume, expressed as a percentage. The terms used in the calculation here are the specific gravity of the component fibres and the weight and thickness of the specimen, which is given by the follow-ing formula [13]:

p = 100 - (dfabric / dfibre)where:p - calculated porosity, dfabric - fabric density and dfibre - fibre density.

Fabric density is expressed in grams per cmł, calculated so that the fabric weight is divided by the fabric thickness.

Pearson correlation coefficients are de-termined in Table 3, and the significance of these coefficients are given under Table 3 by the following procedure as given below:

The value of the random variable tobl is calculated as follows,

where: R – the value of the linear correla-tion coefficient calculated on the basis of the measurement results.

A hypothetical statement about a correla-tion of the variables X and Y in the total general population is in the form of H0: R = 0. If the hypothesis H0 is true, then the computational value of the variable tobl is smaller than the critical value (from the t-Student distribution table, at the sig-nificance level α = 0.05 and the number of the degrees of freedom k = n – 2, that is, tobl < tα). Whereas if: tobl > tα, then it can be accepted that the variables X and Y are correlated, and then the lin-ear correlation coefficient for the total general population is different from zero, that is, R 0. This also means that, for example, feature X (the specific energy) influences the feature Y, such as the area weight thickness, the density, the air permeability, the porosity, the size and distribution of pores, etc. tα is the criti-cal statistical value (selected from the t-Student distribution table, for the sig-nificance level α = 0.05 and the number of the degrees of freedom k = n – 2).

n ResultsTable 2 shows the relationship of the basis weight and thickness change vs. the specific energy given in hydroentangle-ment. We expect that increasing specific energy would decrease the web weight and so the thickness. As may be seen from Table 2 and Table 3, there is no sig-nificant correlation level between these variables in PET fabrics. On the other hand, we obtain significant negative cor-relation between the specific energy and fabric thickness in cotton fabrics. This may be due to the structural differences of the two fibres under discussion. We obtained a more compact and paper-like material in cotton fabrics when we in-creased the energy; however, bulkier fab-rics are produced from PET fibres even at high specific energy levels. This may be

Table 2. Spunlaced nonwoven characteristics.

Cotton PETSpecific energy,

103 x kJkg-1

Basis weight, gm-2

(Standard error)

Thickness, mm (Standard error)

Specific energy,

103 x kJkg-1

Basis weight, gm-2

(Standard error)

Thickness, mm (Standard error)

1.94 51.73 (1.33) 0.84 (0.007) 2.12 47.27 (0.83) 0.823 (0.007) 2.66 47.66 (0.43) 0.68 (0.005) 2.25 51.92 (0.75) 0.873 (0.004) 4.42 48.05 (1.22) 0.67 (0.003) 4.03 51.53 (1.01) 0.890 (0.005) 4.82 44.17 (0.90) 0.73 (0.008) 4.51 47.08 (0.65) 0.713 (0.004) 5.87 44.56 (1.12) 0.66 (0.006) 5.44 49.21 (0.66) 0.850 (0.003) 6.04 53.86 (1.38) 0.76 (0.013) 8.85 36.81 (0.58) 0.610 (0.004) 9.00 48.63 (0.93) 0.66 (0.006) 9.46 46.30 (0.45) 0.631 (0.005)10.30 45.33 (1.27) 0.60 (0.006) 9.88 49.01 (0.92) 0.870 (0.006)12.34 37.97 (0.88) 0.56 (0.008) 11.81 45.91 (0.58) 0.714 (0.004)14.80 37.21 (0.75) 0.58 (0.007) 11.95 46.11 (0.48) 0.614 (0.006)18.53 51.15 (0.92) 0.56 (0.003) 14.80 44.75 (1.21) 0.569 (0.004)22.09 39.13 (0.99) 0.54 (0.006) 18.18 47.27 (0.78) 0.719 (0.004)24.91 49.79 (1.23) 0.55 (0.005) 21.58 47.66 (1.01) 0.772 (0.005)33.82 37.39 (1.53) 0.50 (0.004) 24.55 45.72 (0.66) 0.673 (0.006)45.38 46.88 (1.25) 0.55 (0.003) 33.29 43.78 (0.83) 0.672 (0.004)

- - - 44.99 43.98 (0.71) 0.685 (0.005)

Figure 1. Calculation of maximum pore size and mean pore size (from PMI); according to Akshaya Jena and Krishna Gupta, Liquid Extrusion Techniques for Pore Structure Evaluation of Nonwovens, International Nonwovens Journal, Fall, 2003, pp 45-53.


Table 3. Pearson’s correlation table; * :Correlation is significant at the 0,05 level (2 tailed), ** :Correlation is significant at the 0,01 level (2 tailed).

ParameterCotton PET

Specific energy Air permeability Specific energy Air permeability

Specific energy 1.000 -0.561* 1.000 0.201

Air permeability -0.561* 1.000 0.201 1.000

Basis weight -0.289 -0.339 -0.380 -0.420

Thickness -0.751** 0.275 -0.400 -0.011

Fabric density 0.610* -0.671** 0.146 -0.301

Mean pore size -0.595* 0.776** -0.115 0.632*

Max. pore size -0.180 0.102 -0.065 -0.550*

Min. pore size -0.515* 0.499 -0.210 0.004

Calculated porosity -0.610* 0.671** -0.236 0.351

partly because of the bending behaviours of cotton and PET fibres. It may also result from the possible deformations which occurred in the fibres during the hydroentanglement processes.

In PET fabrics we do not have any sig-nificant correlations between specific en-ergy and other variables or with air per-meability (Figure 2). Since we have used a lab-type hydroentanglement machine, the pressure range was narrower than that of a commercial type, and we could not obtain as high energy for hydroentan-gling PET fibres as we had expected.

Figure 2. Air permeability vs. specific energy in PET fabrics. Figure 3. Mean flow pore diameter vs. specific energy in PET fabrics.

Figure 4. Air permeability vs. specific energy in cotton fabrics. Figure 5. Mean flow pore diameter vs. specific energy in cotton fabrics.

Unlike PET fabrics, we obtain good correlations for cotton fabrics with the experimental data we obtained both for specific energy levels and air permeability (Figure 4). An interesting result shows that air permeability of fabrics decreases as specific energy is increased. Although this may seem irrational, a detailed compara-tive study of the results will reveal that by increasing the specific energy, we decrease both the thickness and the porosity (both minimum and maximum porosity). This re-sult may be explained again by the inherent nature of cotton fabrics. When we increased the energy, we obtained a thinner fabric but

a more paper-like structure, which means that the porosity of the fabric was lost.

If air permeability is compared with the porosity especially, the highest correla-tion among all variables were mean pore size vs. air permeability for both type of fibres (Figure 3 & 5). We get almost the same results as in the literature. Pore structures have a great effect, as was ex-pected. Air permeability is increased as pore size is increased.

n ConclusionsWe investigated several factors related to the effect of the specific energy level and air permeability of fabrics. Differ-ent fibres showed distinct behaviours under almost the same process condi-tions. In general, increasing pressure and the number of passes results in a better structure up to a certain point, provided that the web is sufficiently consolidated. If the web is not consolidated, higher pressures will result in disturbing the web, changing the structure and thus the character of the resulting web.

Porosity and air permeability have a significant correlation, as stated in the literature. We produced bulkier fabrics


Received 10.03.2005 Reviewed 08.11.2005

with PET fibres than with cotton fabrics. Generally speaking, better correlations are obtained for cotton fabrics.

The pilot equipment used for the work can only apply pressures of up to 100 bars, unlike current commercial equip-ment which allows 400 bars or more. Therefore, a further study has been planned to investigate the correlation be-tween specific energy and air permeabil-ity of fabrics from a wide variety of fibres using a commercial production line.

Acknowledgment We express our sincere gratitude to the NC State University and Nonwovens Cooperative Research Centre for their support in using their facilities.

References 1. Subramaniam, V., Madhusoothanan, M.

and Debnath, C. R., Air Permeability of Blended Nonwoven Fabrics, Textile Re-search Journal, 58 (11), 677-678, 1988.

2. Dent, R. W., The Air Permeability of Nonwoven Fabrics, Journal of the Textile Institute, 46(6), 220-224, 1976.

3. Epps, H. H., and Leonas K. K., Pore Size and Air Permeability of Four Nonwoven Fabrics, International Nonwovens Jour-nal, Vol 9, No.2, 2000.

4. Davis, N. C., Factors Influencing the Air Permeability of Felt and Felt-like Struc-tures, Textile Research Journal, 28 (4), 318-324, 1958.

5. Kothari, V. K., Newton, A., The Air Per-meability of Nonwoven Fabrics, Journal of the Textile Institute, 65(8), 525-531, 1974.

6. Atwal, M. S., Factors Affecting the Re-sistance of Nonwoven Needle-punched Fabrics, Textile Research Journal, 57 (10), 574-579, 1987.

7. Medeiros, F. J., Spun-lace/Hydroentan-glement Methods & Products, The Pro-ceedings of Inda-Tech, 1996.

8. Vuillame, A.M., A Global Approach to the Economics and End-product Quality of Spun-lace Nonwovens, Tappi Journal, 149-152, August 1991.

9. Connolly, T. J., Parent, L. R., 1993. Influ-ence of Specific Energy on the Properties of Hydroentangled Nonwoven Fabrics, Tappi Journal 76, No. 8, 135-141.

10. Chowdhury, M. D. H., The Hydrolace Sys-tem and Its Ability to Optimize Modern Fibre Properties in the Development of Unique Properties, Proceedings of Inda-Tech, 1996.

11. Timble, N. B., Allen, C., Hydroentangled Fabric Performance for Polyester/Unbleached Cotton Blends at Various Levels of Specific Energy Levels, The Proceedings of Inda-Tech, 1997.

12. Akshaya Jena Krishna Gupta, Liquid Extrusion Techniques for Pore Structure Evaluation of Nonwovens, International Nonwovens Journal, Fall, 2003, pp. 45-53.

13. Hsieh, Y. L., Liquid Transport in Fabric Structures, Textile Research Journal, 65 (5), 299-307, 1995.

Technical University of ŁódźDepartment of Knitting Technologies

and Structure of Knitted Products

‘TRICOTEXTIL’ Institute of Knitting Technologies and Techniques

Knitt-Tech 20067th INTERNATIONAL

SCIENTIFIC-TECHNICAL CONFERENCE

Subject of the Conference: New Techniques and Technologies in

KnittingThe 7th Knitt-Tech International Scientific-Technical Conference was held from 20 June to 1 July 2006 in Ciechocinek, Poland. This regular/annual event, which has been organised since 1994, was dedicated this time to the theme of ‘New techniques and technologies in knitting’. It was organised by the Department of Knitting Technology and Structure of Knitted Products (DKTSKP) at the Technical University of Łódź (TUŁ) in cooperation with the Tricotextil Institute of Knitting Techniques and Technologies, Łódź.

120 scientists, technologists, and representatives of the industry from Poland, France, Italy, Germany, and India participated in the Conference. The guest lectures of the plenary session were devoted to the following four groups of topics:n Prospects for the textile-clothing branch during globalisation, and the rules

concerning partial financial support for institutions by the European Union. The projects and achievements of the Polish Technological Platform of the Textile Industry after one year/its first year of activity were also listed.

n Modern technologies in knitting, especially concerning knittings for special applications, super-soft cotton knittings manufactured from drawn spinning rowing and ecologically-friendly methods of purifying textile waste-waters.

n Development strategy of the worldwide textile industry, based on examples from Italian and Indian enterprises.

n An outline of education for textile engineers at the Faculty of Textile Engineering and Marketing TUŁ, which considers the current needs of the industry and the research and development centres.

Within the scope of the panel discussion carried out as a part of the LORIS TEX project dedicated to “Transformation of the textile industry from labour-consuming to ‘science-consuming”, the question of prospects for the development of the textile-clothing industry over the next 25 years were discussed.

The poster session of the Conference was dedicated to presenting the results of recent and current research conducted by R&D units, especially those of the Institutes and the DKTSKP.

The conference deliberations were accompanied, as at all of our earlier conferences, by a rich social programme of friendly meetings. Bonfires with singing, including the participation of a professional artistic friendly society, dinner with dancing and a performance by Bogdan Mec, a well-known professional singer, contributed to the integration of the knitting society, and meant that the conference participants left Ciechocinek with good impressions, and in their opinions reinforced the high scientific and organisational levels of the conference.

Technical University of ŁódżFaculty of Textile Engineering and Marketing

Department of Knitting Technologies and Structure of Knitted Products Żeromskiego 116, 90-543 Łódź, Poland

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